Linear accelerators of tandem type



Sept. 28, 1965 F. A. JULIAN ETAL LINEAR ACCELERATORS OF TANDEM TYPEFiled June 2l, 1962 United States Patent O 3,269,269 LINEAR ACCELERATGRSUF TANBEM TYPE Frederick Arthur Julian, Arbortield, and AnthonyChristopher Riviere, Earley, Reading, England, assignors to the UnitedKingdom Atomic Energy Authority, London, England Filed .lune 2l, 1962,Ser. No. 204,163 Claims priority, application Great Britain, .lune 27,1961, 23,308/ 61. 7 Claims. (Cl. S28-233) This invention relates toelectrostatic accelerators of the tandem type. Suitable electrostaticaccelerators, often called Van der Graaf accelerators, are well known.Much information has been given in many publications and detaileddescription is not needed here. Attention is, however, directed to twopublications. The 1958 Accelerator Conference, High Voltage EngineeringCorp., and the Proceedings of the Second Accelerator Conference, NuclearInstruments and Methods, vol. II, No. l, January 1961, in which recentinformation is given.

ln such accelerators negative ions are produced at ground potential andthen accelerated to a high voltage positive terminal. Within theterminal the ions are stripped of electrons and converted to positiveions which therefore can receive further acceleration from the positiveterminal to ground.

It has been proposed already that the negative ions could be given aninitial acceleration before they are injected into the tandemaccelerator, and it has already been suggested that this could beachieved by injecting a neutral particle beam from ground potential intoa lirst tandem accelerator having a negative high voltage terminal atwhich electron addition would occur. The negative particles so producedwould then receive an acceleration from the negative terminal beforebeing injected into the tandem accelerator having a positive centreterminal.

There are, however, ditiiculties inherent in the system alreadyproposed. The minimum gap between the ground potential beam inlet andthe negative voltage terminal is about eight metres and no means has yetbeen found to prevent defocusing of the neutral beam while it drifts,across this gap. Better focusing could be achieved by injecting theneutral particles with higher energy but there is an optimum energy forhighest efficiency of the process producing negative ions at thenegative high voltage terminal, and variation from this optimum wouldreduce the efficiency.

An object of this invention is to provide an electrostatic acceleratorof the tandem type having at least four acceleration stages by the firststage of which a well focused particle beam is provided at the negativehigh voltage terminal.

According to the invention an electrostatic accelerator has a positiveion source adapted to generate heavy molecular positive ions and to beheld at ground potential, a negative high voltage terminal zone adaptedto contain a gas target for the said molecular ions whereby at least aportion of the molecular ions disintegrate into fragments and thefragments pick up electrons to form negative ions, selector means forselecting any desired species from the said negative ions, and apositive high voltage terminal member adapted to accelerate the saidnegative ions and strip electrons from at least a portion thereof toform positive ions.

The fragmentation of the heavy ion will occur by collision with gasparticles moving with thermal energy. The energy of the ion will then bedivided among the fragments. By a suitable choice of variables, e.g.,nature of the ion and accelerating voltage, the energy of the liberatedelement will be such as to optimize the production of negative ions ofthe said element. For a high efiiciency of attachment of electrons to aparticle the Velocity of the particle receiving the electron should becomparable with the orbital velocity of electrons in donor atoms, thatis between l08 and 3X l0a cm./sec. Thus for example in the case ofhydrogen the energy should be between 5 and 40 kev. If the heavy ionsare accelerated to 6 mev. the mass of the heavy ions should be about 150a.m.u. The use of hydrocarbon molecules would enable the liberation ofhydrogen and carbon, and the use of oxygen-containing organic substanceswould allow the liberation of hydrogen, oxygen and carbon at thenegative high voltage terminal. The desired ions for final accelerationcould be selected by magnetic means.

The accelerator of the invention requires a gas target thickness at theelectron attachment canal which would be sufficient to achieve moleculardissociation as well as negative ion formation. The equilibriumthickness for H ion formation at 40 kev. is about 600g Hg cm. and thethickneses required for one mean free path of the dissociation processis 300M Hg cm. if a cross section of 1016 cm.2 is assumed. From thepractical viewpoint a gas target of this thickness is easily achieved.The finite aperture of such a gas target may cause a loss of about 50percent of the negative ions due to the angular spread arising from thedissociation process. However, the problem of focusing the heavymolecular ion beam would be no more dfcult than the injection of H ionsinto a conventional tandem, since the molecular ions would beaccelerated to a high energy in the first stage.

ln a test of the basis of the invention a 3 mev. Van der Graaf singlestage accelerator was used. The radio frequency ion source of thismachine was supplied with hydrogen iodide (molecular Weight 128 a.m.u..)and the accelerator was operated at 1 rnv. The total .ion beam waspassed through a hydrogen gas target between accelerator and analysingmagnet, and the gas pressure adjusted for maximum negative ion yield.Both positive and negative hydrogen ions Were observed whose momentacorresponded to 7.8 kev., which would be expected from the break up ofaccelerated HI+ ions. The negative ion current was about 1011 A. and theratio of positive to negative ion currents was 7 to 1, in reasonableagreement with the equilibrium value of this ratio of 5.7 to 1. Thesmall value of the negative ion current was almost certainly due to thelow yield of HI+ ions from the ion source, and could be raised by theuse of a more efficient source for positive ions. The actual current ofHI+ ions accelerated was not known but if complete dissociation occurredthen the negative ion current would be 2 percent of the incidentmolecular ion current. The choice of an ion containing several hydrogenatoms would increase the number of negative ions available by the sameratio.

An embodiment of the invention is illustrated on the accompanyingdrawing which is a diagrammatic layout.

In the drawing a heavy molecular ion source 1 produces a positive ionbeam 2 which is filtered by a bending magnet 3 and passed into a firsttandem accelerator 4 having a negative high voltage terminal 5 withinwhich is a dissociation and charge exchange canal 6. The negative ionbeam 7 produced therein is passed through a selector magnet 8 and into asecond tandem accelerator 9 having a positive high voltage terminal 10within which is a stripping canal 11. The positive ion beam 12 producedtherein is passed through an analyzing magnet 13 and from thence to atarget 14.

We claim:

1. A method of producing a high-energy beam of positive ions of a givenelement comprising forming positive molecular ions of a compound of thegiven element with at least `one further element, said compound having amolecular weight which is large compared with the atomic weight of saidgiven element; accelerating said molecular ions towards a negativelypolarized terminal Zone containing a gas target adapted to cause atleast a portion of the molecular ions to disintegrate into fragments andthe fragments to pick up electrons to form negative ions, acceleratingsaid negative ions beyond said negatively polarized terminal zone,selecting ions of the given element from said accelerated negative ions,accelerating said selected negative ions towards a positively polarizedterminal zone containing means for stripping electrons from at least aportion of said selected ions to form positive ions of the givenelement, and accelerating said positive ions beyond said positivelypolarized terminal zone.

2. A method of producing a high-energy beam of positive hydrogen ionscomprising forming positive molecular ions of a compound of hydrogenwith at least one further element, said compound having a molecularweight which is large compared with the atomic weight of hydrogen,accelerating said molecular ions towards a negatively polarized terminalzone containing a gas target adapted to cause at least a portion of themolecular ions to disintegrate into fragments and the fragments to pickup electrons to form negative ions, accelerating said negative ionsbeyond said negatively polarized terminal zone, selecting hydrogen ionsfrom said accelerated negative ions, accelerating the selected hydrogenions towards a positively polarized terminal zone containing means forstripping electrons from at least a portion of the hydrogen ions to formpositive hydrogen ions, and accelerating the positive hydrogen ionsbeyond said positively polarized terminal zone.

3. A method as claimed in claim 2 wherein said compound is hydrogeniodide.

4. An electrostatic accelerator comprising a positive ion source, saidsource generating an ion beam of positive molecular ions of at least onedesired element, the molecular weight of said positive molecular ionsbeing large compared with the atomic weight of said desired element, anegatively polarized zone for accelerating said positive molecular ionsaway from said source a'nd toward the negative zone, duct meansproviding an uninterrupted path for said heavy positive molecular ionsfrom said source to said zone, dissociation and charge exchange meansWithin said negative zone, said dissociation and charge exchange meanscomprising a target of gas particles for collision with said positivemolecular ions for liberating positive atomic ions of the elements ofsaid positive molecular ions and for imparting thereto electrons to formnegative atomic ions, the velocity of said positive molecular ions beingsuiliciently low at the point of collision with said gas particles to becomparable to the velocity of the electron donor atoms of said gasparticles, thereby achieving optimum eiiiciency in the formation ofnegative atomic ions, selector means for selecting the negative atomicions of said desired element, and positive high voltage terminal meansfor accelerating the negative atomic ions of said desired element andmeans for stripping electrons from said negative atomic ions to formpositive atomic ions.

5. An accelerator according to claim 4 wherein said positive molecularions containing said desired element are of a chemical compound.

6. An accelerator according to claim 4 wherein said selector meanscomprises a selector magnet of suiiicient field strength to bend thenegative ions of said desired element along a predetermined path.

'7. An electrostatic accelerator as claimed in claim 4 in which thenegative high potential terminal zone is within a rst pressure vesseladapted to be held at earth potential and the positive high voltageterminal is within a second pressure vessel adapted to be held at earthpotential.

References Cited by the Examiner Neutralization and Ionization etc. byBatho, December 15, 1932, Physical Review, vol. 42, page 755.

Negative Hydrogen Ion Source by Weinman and Cameron, May 1956, Review ofScientific Instruments, vol. 27, No. 5, pages 288-293.

Nuclear Instruments and Methods, vol. 11, No. 1,

January 1961, page 109.

Zhurnal Eksperimental noi iTeoreticheskoi Fiziki V 34n6, June 1958, pp.1398-1405.

GEORGE N. WESTBY, Primary Examiner.

DAVID I. GALVIN, Examiner.

1. A METHOD OF PRODUCING A HIGH-ENERGY BEAM OF POSITIVE IONS OF A GIVENELEMENT COMPRISING FORMING POSITIVE MOLECULAR IONS OF A COMPOUND OF THEGIVEN ELEMENT WITH AT LEAST ONE FURTHER ELEMENT, SAID COMPOUND HAVING AMOLECULAR WEIGHT WHICH IS LARGE COMPARED WITH THE ATOMIC WEIGHT OF SAIDGIVEN ELEMENT; ACCELERATING SAID MOLECULAR IONS TOWARDS A NEGATIVELYPOLARIZED TERMINAL ZONE CONTAINING A GAS TARGET ADAPTED TO CAUSE ATLEAST A PORTION OF THE MOLECULAR IONS TO DISINTEGRATE INTO FRAGMENTS ANDTHE FRAGMENTS TO PICK UP ELECTRONS TO FORM NEGATIVE IONS, ACCELERATINGSAID NEGATIVE IONS BEYOND SAID NEGATIVELY POLARIZED TERMINAL ZONE,SELECTING IRONS OF THE GIVEN ELEMENT